(1) Field of the Invention
This invention relates to a speech-amplifier telephone station which is more generally known as a "hands-free" telephone station.
In hands-free telephone stations, a highly sensitive microphone and a loudspeaker associated with high-gain amplifiers are integrated with the telephone housing, thus making it unnecessary for a microphone transmitter and receiver to be held to the user's mouth and ear. The convenient features and ease of use offered by subscriber stations of this type are offset, however, by a few constraints which are caused by the following phenomena.
Because they are high-gain amplifiers in telephone stations of this type, the acoustic coupling which arises between the loudspeaker and the microphone very rapidly results in the appearance of a whistling noise which is known as the Larsen effect and makes conversation practically inaudible.
Similarly, because a circuit is provided within the telephone station for coupling it with the telephone line and switching the signal received from the line to the loudspeaker, the signal transmitted by the microphone to the telephone line produces a capacitive coupling between the transmission and reception channels as a result of the unbalance introduced by the line at the level of the device for differential coupling of these two channels. By reason of the presence of high-gain amplifiers, the capacitive coupling just mentioned also causes whistling noise.
(2) Description of the Prior Art
In order to eliminate this unpleasant whistling noise, a known practice consists of interrupting the loop which permits sustained oscillations caused by Larsen effect or microphonic noise. This is done either by interrupting the connection between the coupling circuit and the loudspeaker or by interrupting the connection between the microphone and the coupling circuit, depending on whether the user is talking or listening. Under these conditions, a hands-free subscriber station achieves reproduction of speech with satisfactory fidelity only if the two correspondents speak in alternate sequence and not simultaneously. The interruption can be replaced by very high attenuation, which considerably limits the consequences of the Larsen effect and does not completely exclude the possibility of intervention by the correspondent.
A hands-free station of this type is described in French patent Application No. 79 07 109 French Pat. No. 2,452,215.
This station includes two switches, one placed on the reception channel between the coupling circuit and the loudspeaker, the other or the transmission channel between the microphone and the coupling circuit. Control of these switches is based on a comparison of the signals collected upstream of the switches, on the transmission channel between the microphone and the switch associated with that channel, and on the reception channel between the coupling circuit and the switch associated with that channel.
Because the compared signals are collected upstream of the switches, the Larsen effect is taken into account in this comparison. It must be insured, however, that the presence of the Larsen effect upstream of the switch on the interrupted channel is not interpreted as an indication of the presence of a speech signal and accordingly countermands the decision to close the switch on the other channel since this would result in chopping of the communication.
A known expedient proposed by the patent application cited earlier is collecting the same sign quantity on both channels as long as none of the switches has been closed, and then introducing a difference in the signal quantity collected on both channels when one of the switches has been closed.
The aforementioned patent Application proposes to control one of the switches by first making a comparison between the signal collected from the transmission channel and. The signal which is collected from the reception channel and to which a multiplication factor is assigned in the event of a signal which is present on both channels; thus multiplication factor is made dependent on the value of the signal obtained on completion of the first comparison. In order to control the other switch, a second comparison is made between the signal collected from the reception channel and the signal which is collected from the transmission channel and to which a multiplication factor is assigned in the event of a signal which is present on both channels. This multiplication factor is made dependent on the value of the signal obtained on completion of the second comparison.
The complexity of this method for processing collected signals requires that speech signals be converted to binary data, which is turn calls for a complex control logic system and for insertion for a delay line between the point of collection of the signal on each channel and the switch associated with each channel. The time-delay produced by this delay line is equal to the time of processing of the collected signals.